1. Field of the Invention
The present invention relates to a hydraulic driving apparatus provided in a working machine, such as a crane, to move a load, such as a suspended load, in the same direction as a self-weight falling direction of the load.
2. Description of the Background Art
As a hydraulic driving apparatus for moving a load in the same direction as a self-weight falling direction of the load, there is known, for example, a lowering drive apparatus for driving a winch suspending a suspended load by a wire rope, in a lowering direction. In this apparatus, it is important to prevent that an excessive decrease in pressure on a meter-in side during a lowering drive causes cavitation which stalls winch driving to thereby bring the suspended load into free falling.
As means to prevent such a decrease in pressure on the meter-in side, JP 2000-310201A describes a so-called external pilot-operated counterbalance valve provided in a flow passage on a meter-out side. The external pilot-operated counterbalance valve is operable to narrow the flow passage on the meter-out side when the pressure on the meter-in side becomes equal to or less than a setting pressure thereof, thereby preventing the pressure on the meter-in side from an excessive decrease.
The external pilot-operated counterbalance valve, however, has a pressure measurement point on the meter-in side while having a pressure control point thereof on the meter-out side; that is, the external pilot-operated counterbalance valve is configured to perform control under the condition that positions of measurement and control points are different from each other, i.e., perform control out of so-called control-theoretic co-location, thus having a problem that the performed control is fundamentally unstable and likely to involve hunting.
As means to prevent the above hunting, there exists a technique of providing an orifice capable of giving large attenuation to an opening movement of the counterbalance valve, in a pilot fluid passage; however, this technique has a problem that the orifice prolongs a valve opening time of the counterbalance valve to deteriorate the responsiveness of the counterbalance valve, and further provides the counterbalance valve with a large flow resistance until it is fully opened to thereby generate an unnecessary boosted pressure.
As another technique for preventing the hunting, the JP 2000-310201A describes a communication valve for controlling fluid communication between the flow passage on the meter-in side and the flow passage on the meter-out side, and a flow adjustment valve for controlling a meter-in flow rate to reduce a pressure difference between the two flow passages; however, this technique has difficulty in obtaining a stable lowering speed. In a lowering control circuit, generally, there is generated a holding pressure corresponding to a weight of a suspended load on a meter-out side, so that, the larger the weight of the suspended load, the larger the pressure difference between meter-out and meter-in sides becomes, and the increase in the pressure difference involves an increase in an opening degree of the flow adjustment valve, thus increasing the meter-in flow rate. Hence, the above conventional apparatus has a possibility of large variation in the lowering speed depending on a level of the load.